1. Technical Field
The present invention relates to a method of molding and baking polyphenylene sulfide (PPS) for overmold carriages for use in computer hard disk drives to minimize heat-induced deflections and improve structural stiffness characteristics.
2. Description of the Related Art
Carriage assemblies in computer hard drives carry read/write transducer heads across the surface of the disks to the desired information track. The read/write heads are positioned in very close proximity to the surface of the disks, riding upon an air bearing created by the rapid rotation of the disk. Through accompanying electronic circuitry, the read/write heads are used to read digital information from the disks and to write digital information to the disks. It is known in the art to form these carriage assemblies from lightweight, strong materials and from combinations of the materials. These materials are primarily metals, such as aluminum, and plastics, such as polyphenylene sulfide (PPS) with 40% glass fill.
PPS is a thermoplastic polycondensate that has been mass-produced since 1973 and is sold by Chevron Phillips Chemical Company under the trademark RYTON. PPS is used to make high performance electrical connectors and other high stress parts because of its high temperature resistance (230xc2x0 C. continuous) and low temperature endurance.
FIG. 1 shows a typical overmold carriage assembly 11, assembly 11 having two arms 13 extending from a molded plastic body 15. Arms 13 are preferably formed from stamped aluminum and have forward ends 17 that are adapted to carry read/write heads (not shown). Body 15 is formed by molding PPS 40% glass fill on a carriage in an injection mold, body 15 having a central portion 19 and a rear portion 21. An opening 23 in central portion 19 acts a pivot center for assembly 11. Rear portion 21 houses a voice coil 25 and may house other components of the carriage assembly.
Typically, overmold carriage manufacturers inject molten PPS 40% glass fill into a mold preheated to approximately 80xc2x0 C. Because the glass transition temperature of PPS 40% glass fill is between approximately 80xc2x0 C. and approximately 125xc2x0 C., less warpage and plastic deformation occurs if the mold is heated above 130 deg C. or below 80 deg C. To allow for the use of lower mold temperatures, carriage manufacturers will typically post-bake the finished carriage at a higher temp, for example, approximately 130xc2x0 C., to improve the characteristics of the finished part. An alternate prior-art method of forming overmold carriages uses a mold temperature of approximately 130xc2x0 C., then the carriage is post-baked at approximately 80xc2x0 C.
Overmold carriages molded using this method have been shown to undergo a severe deflection in the range of approximately 0.03 mm to 0.04 mm as they are exposed to the higher temperatures for as little as 1 to 2 hrs, limiting the use of PPS 40% glass fill to designs with loose tolerances and thus non-high performance designs. FIG. 2 is a graph of the deflection vs. number of thermal cycles in samples of PPS 40% glass fill molded using the alternate 130xc2x0 C. molding process with a post bake at 80xc2x0 C. FIG. 2 shows that the average deflection increases with each successive thermal cycle, and these results are comparable to those achieved when using the method using a 80xc2x0 C. mold and a 130xc2x0 C. post-bake.
An additional problem with the typical prior-art method is that the material produced has a wide range of structural stiffness. The structural stiffness determines the frequency response of the material, and resonance modes can shift by as much as 200 Hz, causing problems with servo design and performance. FIG. 3 is a frequency response plot for a carriage assembly produced using the typical prior-art method. Butterfly modes, referring to the in-plane bending of the rear portion of the body that houses the coil, occur at approximately 5.25 kHz, approximately 12.0 kHz, and approximately 17.0 kHz. FIG. 3 also shows the variation in frequency responses of different samples, especially at higher frequencies.
Therefore, there is a need for an improved method of forming carriage assemblies from PPS 40% glass fill in which the resulting parts have improved resistance to deflection during heating and improved structural stiffness characteristics.
A method is provided for forming a body for a hard disk carriage assembly from polyphenylene sulfide 40% glass fill. An injection mold is heated to approximately 130xc2x0 C.; then a volume of polyphenylene sulfide 40% glass fill is injected into the mold to form the body of the carriage assembly. The polyphenylene sulfide 40% glass fill is then cooled within the mold for approximately 45 seconds before the body is ejected from the mold cavity. The body is then heated to approximately 130xc2x0 C. for at least 2 hours in a post-bake operation.